A flyback switching power converter is commonly used to charge mobile devices as the converter's transformer provides safe isolation from AC household current. It is conventional for the switching power converter to couple to the mobile device being charged through a standard interface such as a Universal Serial Bus (USB) interface. With regard to the delivery of power, a USB cable can only provide a certain amount of current. For example, the USB 2.0 standard allows for a maximum output current of 500 mA whereas the USB type C Power Delivery (PD) standard allows a maximum output current of 5 A (depending upon the particular cable configuration) over a Vbus terminal. With the delivery of so much power, the USB type C protocol requires the power converter supplying power to the USB cable to include a Vbus switch that when closed isolates the power converter output from the Vbus terminal.
The operation of the Vbus switch depends upon whether the upstream facing port (UFP) of the USB cable is connected to a mobile device for receiving power. The power converter connects to the downstream facing port (DFP) of the USB cable. To detect the connection of the mobile device to the UFP, the power converter monitors the voltage of a configuration channel (CC) terminal at the DFP. When connected, the mobile device causes the CC terminal at the DFP to discharge for a de-bouncing period. The power converter reacts to the expiration of the de-bouncing period by closing the VBus switch from its default open state so that power may be supplied over the Vbus terminal. Although the resulting high power delivery over the USB cable is thus advantageous, problems have arisen with regard to its implementation. For example, the USB interface may get dirty such that a dust particle or other slightly conductive object couples between the Vbus pin (the pin delivering the output voltage) and one of the signaling or ground pins. Alternatively, the USB cable itself may become frayed from twisting by a user such that a slightly conductive path exists between the Vbus wire and one of the remaining wires. The result is either a “hard short” or a “soft short” between the Vbus terminal and one of the remaining USB terminals. As compared to a hard short, a soft short has a relatively high impedance between the corresponding pins (or wires) in the USB cable or interface.
An example USB type C system 101 is shown in FIG. 1. A flyback switching power converter 100 includes a primary-side controller 125 and a secondary-side controller 120. Primary side controller 125 regulates the switching of a power switch transistor S1 to provide a 5.0V Vbus voltage for the charging of a Vbus terminal in a USB connector 110 for USB cable 115. Power switch transistor S1 is in series with a primary winding T1 of a transformer having a secondary winding T2. While power switch transistor S1 conducts in response to being switched on by controller 125, an output diode coupled to the secondary winding T2 is reverse-biased and thus non-conducting. But when power switch transistor S1 opens, the voltage reverses across secondary winding T2 such that the output diode D1 becomes forward-biased and conducting. The resulting charge cannot flow into the Vbus terminal while a Vbus switch transistor S2 is non-conducting. To allow reversibility of the cable connections, USB connector 110 includes two configuration channel terminals: a CC1 terminal and a CC2 terminal. Depending upon the connection orientation, a client device 105 will discharge one of the CC terminals. Secondary controller 120 reacts to this discharge by closing Vbus switch transistor S2 after the de-bouncing period. But note that output capacitor C1 is charged to the 5.0 V power supply voltage. Should a short circuit 140 provide a conductive path from the Vbus terminal to a ground (GND) terminal, output capacitor C1 will discharge a short circuit current through Vbus switch transistor S2 and short circuit 140.
The resulting short circuit waveforms for system 101 are shown in FIG. 2. The CC1 pin voltage discharges at a time t0 due to the connection of client device 105 to USB cable 115. After expiration of the de-bouncing period, secondary-side controller 120 switches Vbus switch transistor S2 on by pulsing its gate voltage high at a time t1. The Vbus terminal voltage rises in response the Vbus switch transistor S2 switching on. Due to the presence of short circuit 140, the output current (I_discharge) through the Vbus switch transistor S2 pulses above its maximum current rating. This relatively large output current quickly pulls the power supply voltage (VCC) stored across output capacitor C1 (VCC) below a primary-side controller reset threshold at a time t2. Primary-side controller 125 then resets such that secondary-side controller 120 then re-asserts the Vbus switch transistor gate voltage at a time t3. The output current again spikes and the resulting drop in VCC causes another reset, and so on. This repetitive exposure to such large output currents eventually causes Vbus switch transistor S2 to fail and assume an irreversible constant-on state or an irreversible constant-off state. Both of these states are of course undesirable.
Accordingly, there is a need in the art for improved short circuit protection for switching power converters that charge over data interfaces.